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Traditional photovoltaic cells turn a relatively small part of the sun’s light spectrum into electricity, limiting their efficiency and power output. The cell’s silicon material responds to a limited range of light wavelengths, ignoring those that are longer and shorter. As the wavelength varies from short to long, the cell’s output rises and falls in a jagged curve. Newer photovoltaic cell designs achieve higher efficiency by converting more wavelengths into useful energy.

Light Spectrum and Wavelength

Visible light is a very small part of the electromagnetic spectrum, a continuous range of energy wavelengths that includes radio waves, light and X-rays. Visible light waves measure between 400 and 700 nanometers, although the sun’s spectrum also includes shorter ultraviolet waves and longer waves of infrared. A photovoltaic cell responds selectively to light wavelengths. Those much longer than 700 nanometers lack the energy to affect the cell and simply pass through it. Very short wavelengths, such as X-rays, pass through the cell because their energy is too high to be absorbed.

Cell Use of Light Energy

The silicon atoms in a photovoltaic cell absorb energy from light wavelengths that roughly correspond to the visible spectrum. The cell has silicon mixed with two different impurities that produce positive and negative charges. Light causes the charges to move, producing an electric current. Materials containing different impurities change the wavelengths at which the cell responds in different ways. The photovoltaic cell doesn’t convert all the light, even if it’s at the right wavelength. Some of the energy becomes heat, and some reflects off the cell’s surface.

Photovoltaic Response Curve

If you carefully plot a solar cell’s output energy against the wavelength of incoming light, your graph will show a response curve that begins at about 300 nanometers. It arrives at a maximum at about 700 nanometers, makes a series of peaks and dips, and falls abruptly at 1,100 nanometers -- the maximum wavelength for silicon. Quantum effects in the material account for the bumpy nature of the curve because the silicon atoms respond efficiently to some wavelengths, and less efficiently to adjacent ones.

High Efficiency Photovoltaic Cells

To increase the efficiency of photovoltaic cells, materials engineers have adopted a variety of techniques, including a multi-layer design that has several types of impurities mixed with the silicon, each with its own response curve. The top layer absorbs shorter wavelengths and the bottom converts the longer ones. The result is significantly better conversion efficiency and better energy output.

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About the Author

Chicago native J.T. Barett has a Bachelor of Science in physics from Northeastern Illinois University and has been writing since 1991. He has contributed to "Foresight Update," a nanotechnology newsletter from the Foresight Institute. He also contributed to the book, "Nanotechnology: Molecular Speculations on Global Abundance."